Testing the Fidelity of Methods Used in Proxy-Based Reconstructions of Past Climate (original) (raw)
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2005: Testing the fidelity of methods used in proxy-based reconstructions of past climate
2014
Two widely used statistical approaches to reconstructing past climate histories from climate 'proxy ' data such as tree-rings, corals, and ice cores, are investigated using synthetic 'pseudoproxy ' data derived from a simulation of forced climate changes over the past 1200 years. Our experiments suggest that both statistical approaches should yield reliable reconstructions of the true climate history within estimated uncertainties, given estimates of the signal and noise attributes of actual proxy data networks. 1.
Climate Dynamics, 2006
We present and describe in detail the advantages and limitations of a technique that combines in an optimal way model results and proxy-data time series in order to obtain states of the climate system consistent with model physics, reconstruction of past radiative forcing and proxy records. To achieve this goal, we select among an ensemble of simulations covering the last millennium performed with a low-resolution 3-D climate model the ones that minimise a cost function. This cost function measures the misfit between model results and proxy records. In the framework of the tests performed here, an ensemble of 30 to 40 simulations appears sufficient to reach reasonable correlations between model results and reconstructions, in configurations for which a small amount of data is available as well as in data-rich areas. Preliminary applications of the technique show that it can be used to provide reconstructions of past large-scale temperature changes, complementary to the ones obtained by statistical methods. Furthermore, as model results include a representation of atmospheric and oceanic circulations, it can be used to provide insights into some amplification mechanisms responsible for past temperature changes. On the other hand, if the number of proxy records is too low, it could not be used to provide reconstructions of past changes at a regional scale.
The Value of Multiproxy Reconstruction of Past Climate
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Understanding the dynamics of climate change in its full richness requires the knowledge of long temperature time series. Although long-term, widely distributed temperature observations are not available, there are other forms of data, known as climate proxies, that can have a statistical relationship with temperatures and have been used to infer temperatures in the past before direct measurements. We propose a Bayesian hierarchical model to reconstruct past temperatures that integrates information from different sources, such as proxies with different temporal resolution and forcings acting as the external drivers of large scale temperature evolution. Additionally, this method allows us to quantify the uncertainty of the reconstruction in a rigorous manner. The reconstruction method is assessed, using a global climate model as the true climate system and with synthetic proxy data derived from the simulation. The target is to reconstruct Northern Hemisphere temperature from proxies that mimic the sampling and errors from tree ring measurements, pollen indices and borehole temperatures. The forcing series used as covariates are solar irradiance, volcanic aerosols and green house gas concentrations.
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Curated global climate data have been generated from climate model outputs for the last 120,000 years, whereas reconstructions going back even further have been lacking due to the high computational cost of climate simulations. Here, we present a statistically-derived global terrestrial climate dataset for every 1,000 years of the last 800,000 years. It is based on a set of linear regressions between 72 existing HadCM3 climate simulations of the last 120,000 years and external forcings consisting of CO 2 , orbital parameters, and land type. The estimated climatologies were interpolated to 0.5° resolution and bias-corrected using present-day climate. The data compare well with the original HadCM3 simulations and with long-term proxy records. Our dataset includes monthly temperature, precipitation, cloud cover, and 17 bioclimatic variables. In addition, we derived net primary productivity and global biome distributions using the BIOME4 vegetation model. The data are a relevant source ...
Climatic Change, 2011
Reliable paleoclimate reconstructions are needed to assess if the recent climatic changes are unusual compared to pre-industrial climate variability. Here, we focus on one important problem in climate reconstructions: Transfer functions relating proxies (predictors) and target climatic quantities (predictands) can be seriously biased when predictand and predictor noise is not adequately accounted for, resulting in biased amplitudes of reconstructed climatic time series. We argue for errors-in-variables models (EVM) for unbiased identification of linear structural relationships between noisy proxies and target climatic quantities by (1) introducing underlying statistical concepts and (2) demonstrating the potential biases of using the EVM approach, the most commonly used direct ordinary least squares (OLS) regression, inverse OLS regression, or the reduced major axis method ('variance matching') with a simulation example of artificial noise-disturbed sinusoidal time series. We then develop an alternative strategy for paleoclimate reconstruction from tree-ring and other proxy data, explicitly accounting for the identified problem.
Climate of the Past, 2013
The mid-Holocene (6 thousand years before present) is a key period to study the consistency between model results and proxy data as it corresponds to a standard test for models and a reasonable number of proxy records are available. Taking advantage of this relatively large amount of information, we have first compared a compilation of 50 air and sea surface temperature reconstructions with the results of three simulations performed with general circulation models and one carried out with LOVECLIM, a model of intermediate complexity. The conclusions derived from this analysis confirm that models and data agree on the large-scale spatial pattern but underestimate the magnitude of some observed changes and that large discrepancies are observed at the local scale. To further investigate the origin of those inconsistencies, we have constrained LOVECLIM to follow the signal recorded by the proxies selected in the compilation using a data assimilation method based on a particle filter. In one simulation, all the 50 proxies are used while in the other two, only the continental or oceanic proxies constrains the model results. This assimilation improves the consistency between model results and the reconstructions. In particular, this is achieved in a robust way in all the experiments through a strengthening of the westerlies at mid-latitude that warms up the Northern Europe. Furthermore, the comparison of the LOVECLIM simulations with and without data assimilation has also objectively identified 16 proxies whose reconstructed signal is either incompatible with the one recorded by some other proxies or with model physics.
Updating historical tree-ring records for climate reconstruction
Quaternary Science Reviews, 2010
Over the past three decades, numerous Late Holocene-long tree-ring (TR) chronologies have been developed for different parts of Europe that allow archaeological, historical and cultural wood remains to be dated with annual precision. Ironically, palaeoclimatic evidence inherent in such composites is limited as modern updates essential for calibration/verification with instrumental measurements are often inappropriate, incomplete or even missing. Here we proposes a novel approach to updating historical TR records while preventing statistical over-fit with the target data and advocate 'horizontal' splitting between historical (early) and recent (modern) TR samples prior to their standardization (detrending). This split-technique will help to overcoming unprecedented effects of increased atmospheric greenhouse-gas, biospheric fertilization, forest management, sample replication, age-structure and chronology development associated with modern proxy updates.
Millennial reconstruction of the global terrestrial climate: New approaches to the available data
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The main aim of the work was to reliably reconstruct the change in global temperature for the last 1000 years using new methods. We have considered two approaches to obtaining composite series based on symbolic analysis and on the so called "principle of witnesses," respectively. Based on the first approach, it has been indicated that the known reconstruction proposed by Moberg et al. is preferable in the scope of the accepted symbolic analysis variant among the available reconstructions. An approach based on the eyewitness principle made it possible to reveal the years when five different reconstructions give coordinated data on a 1000 year scale. The obtained new reconstructions are in good agreement with instrumental series data on the consistent time interval of their existence.